1. Field of the Technology
The present invention relates to the simultaneous measurement of target velocity and position by microwaves (radar), sound (sonar and ultrasound) and light, and more specifically, it relates to the use of long delay interferometric techniques, short coherence length illumination and the Doppler effect to measure velocity and range.
2. Description of the Background
Measuring the velocity and range of an object remotely by the response of reflected waves (microwaves, light and sound) is an important diagnostic tool in a variety of fields in science and engineering. In many cases, the velocimetry and range finding are accomplished by opposite kinds of illuminating waves. Traditionally, highly coherent and therefore narrowbanded illumination is used for velocimetry, so that the Doppler frequency shift in the reflected waves is larger than the illumination bandwidth .beta.. This allows the Doppler frequencies of moving targets to be easily distinguished from those of stationary objects by the use of narrowbanded filters. In contrast, for range finding a short coherence length .LAMBDA. is desired, since half the coherence length defines the range resolution. A short coherence length implies broadband illumination because of the reciprocal relationship .LAMBDA..about.c/.beta., where c is the speed of the waves. Thus, illumination optimized for conventional velocimetry is very different from that optimized for range finding.
For example, monochromatic laser illumination is conventionally used for optical velocimetry, whereas broadband and incoherent light such as white light is used for rangefinding, which in optics is called optical coherence tomography. Pulse Doppler radars are used for velocimetry having monochromatic waves enveloped into .about.1 .mu.s pulses. The range resolution of these is limited by the .about.300 meter pulse envelope length. In contrast, range finding or mapping radars use illumination having a much shorter coherence length. This is often achieved by increasing the bandwidth of the waveform through frequency modulation or the repetitive use of a coded pulse to simulate randomness.
A theoretical optimum waveform for many range finding applications is wideband and incoherent, analogous to white light generated by incandescence. For radar and sonar this could be generated by an electronic noise source. A truly incoherent waveform is random and therefore never repeats. This is an advantage because any speckle or temporary coherences in the echolocation target image is blurred away, improving the observation of detail. Incoherent illumination will not suffer the interference effect between the transmitted beam and parasitic reflections from the Earth, thus minimizing the lobed characteristics of antennas in the altitude direction. A further advantage of incoherent illumination over a short pulse or frequency modulated pulse having the same coherence length is that a random waveform can be arbitrarily long without repetition. That is, its time-bandwidth product can be arbitrarily large. The longer pulse carries correspondingly more energy to the target, which increases the maximum range of detection.
An important desirable trait of echolocation systems is the ability to measure velocity and range simultaneously to high resolution. In radar, the velocity resolution is used to separate moving targets from ground clutter. Once detected, the range and if possible, the size of the target is desired to be known. In medical ultrasound, the motion of the blood can distinguish blood vessels from surrounding tissues. A map of the vessels having high spatial resolution and color coded by velocity is the goal of these devices.
Measuring the velocity and range simultaneously to high resolution is possible by measuring the range to high resolution at two different, but well defined times and finding the rate of change. This is accomplished in some radars using short coherence waves and by cross correlating the reflected and transmitted signals. A disadvantage with a cross-correlation is that distortions in the waves suffered anywhere between the transmitter and receiver can broaden the cross-correlation peak, and hence can blur the velocity determination. Such distortions could include distortions in the final stages of the transmitting amplifier, the effect of propagation through the atmosphere, the target albedo spectrum such as resonances, and reverberations from previous pings.
A different method for determining velocity is the autocorrelation. This is a comparison of the received signal with itself delayed by a time .tau..sub.2. For those waveforms which produce an autocorrelation peak, the peak shifts with velocity v by an amount .DELTA..tau..sub.2 =2.tau..sub.2 v/c. The advantage of the autocorrelation is that it will not broaden by slowly varying distortions, which includes many practical distortions. Thus the autocorrelation will give an accurate velocity under distortions that would cause a cross-correlation to blur.
The problem is that autocorrelations cannot be used with the most desirable illumination, perfectly random and ever changing waveforms, because these waveforms are by definition incoherent over all delay scales. The method of the present invention is a solution to this problem. In addition to velocimetry using incoherent illumination, the invention is capable of measuring range and velocity simultaneously to high resolution, providing a way to discriminate moving targets from background clutter.